1. Field of the Invention
This invention relates to a printer wherein ink on an ink ribbon accommodated in a ribbon cassette is transferred to paper by an action of a print head such as a wire dot head or a thermal head, and more particularly to a paper feed roller arrangement for feeding paper in such printer.
2. Description of the Prior Art
One of such printers as described above wherein printing is accomplished with an ink ribbon has a construction as shown in FIG. 3 wherein paper P is fed in a horizontal plane.
General construction of a conventional printer of this type will be described with reference to FIG. 3. A platen 1 in the form of a flat plate extends parallel to a feeding direction of paper P and is disposed such that a pressure receiving face 2 thereof is directed upward as seen in FIG. 3. A carriage not shown, is mounted for movement in a direction parallel to the platen 1, and a wire dot head 3 is carried on the carriage and disposed above the platen 1 in an opposing relationship to the platen 1. A ribbon cassette 4, shown in phantom in FIG. 3, is also carried on the carriage, and an ink ribbon 5 is accomodated in the ribbon cassette 4 and has an intermediate portion thereof disposed between the platen 1 and the wire dot head 3.
The paper P is fed in the leftward direction as indicated by an arrow mark in FIG. 3 in the horizontal plane between the platen 1 and the wire dot head 3. A pair of paper feed rollers 6A and 6B and another pair of paper feed rollers 7A and 7B for feeding the paper P in the direction indicated by the arrow mark are disposed on the upstream side and the downstream side, respectively, of the wire dot head 3 in the feeding direction of the paper P such that they are in contact with the paper P from above and below. The lower paper feed rollers 6B and 7B have a greater diameter and serve as driving rollers while the upper paper feed rollers 6A and 7A have a smaller diameter and serve as driven or follower rollers.
A conventional printer has a construction as described above. Thus, in order to accomplish printing, the paper feed rollers 6B and 7B are driven to feed the paper P between the paper feed rollers 6A and 6B and between paper feed rollers 7A and 7B a predetermined distance in the directions indicated by the double-sided arrow mark and then stopped at a predetermined location of the paper P which is opposed to the wire dot head 3. In this condition, the carriage, not shown, is actuated to transfer ink on the ink ribbon 5 to the paper P by means of the wire dot head 3 to effect serial printing. Next, the paper feed rollers 6B and 7B are driven to feed the paper P by a distance equal to a one line space and then stopped, whereafter the carriage is actuated to begin serial printing again.
When the ribbon cassette 4 is to be replaced it must be moved substantially in a horizontal direction. Since the direction of such movement coincides with a direction indicated by the double-sided arrow mark in FIG. 3, and since the ribbon cassette 4 is disposed at a location comparatively near the wire dot head 3, a portion of the ink ribbon 5 exposed to the lower end of the ribbon cassette 4 is likely to be contacted with the paper feed roller 7A or 6A. These rollers oppose the passage of the ribbon cassette 4 when a replacement operation of the ribbon cassette 4 is performed. Accordingly, the ribbon cassette 4 must be moved with extreme care so that the ribbon 5 will not contact the paper feed roller 7A, or 6A which makes exchanging of the ribbon cassette 4 cumbersome. Further, if the ink ribbon 5 is contacted with the paper feed roller 7A or 6A, the ink of the ink ribbon 5 is likely to adhere to the paper feed roller 7A. Consequently, the ink may be transferred from the paper feed roller 7A or 6A to the paper P being fed and the paper P may be soiled by the ink.
An improved printer as shown in FIG. 4 which eliminates the drawbacks of the conventional printer described above with reference to FIG. 3 is also known conventionally. In the improved printer shown in FIG. 4, a paper feed roller 7A and a cooperating paper feed roller 7B for are spaced by a greater distance from a wire head 3 than that in the printer shown in FIG. 3.
In the printer shown in FIG. 4, however, the distance between a pair of paper feed rollers 6A and 6B and the paper feed rollers 7A and 7B is correspondingly greater than that in the printer shown in FIG. 3. Accordingly, in the printer of the type wherein printing cannot be performed unless the paper P is held in a taut condition by the paper feed rollers 6A and 6B and the other paper feed rollers 7A and 7B, a significantly large spacing or margin remains on the leading end side and the trailing end side of the paper P. Thus the waste of the paper P is significant.